diff --git a/trac/mimeview/pygments.py b/trac/mimeview/pygments.py
index c3fa038346..7e9cabf87d 100644
--- a/trac/mimeview/pygments.py
+++ b/trac/mimeview/pygments.py
@@ -174,7 +174,9 @@ def style_defs(style):
         if default_style not in styles:
             default_style = 'trac'
         selection = req.session.get('pygments_style')
-        if selection not in styles:
+        if not selection:
+            selection = ''
+        elif selection not in styles:
             selection = default_style
         output = self._generate('html', self.EXAMPLE)
         add_script_data(req, default_style=default_style, selection=selection)